You won't need 300dpi or 150dpi and certainly not 3000dpi (I guess that was a typo).
For something that large, you can reduce the print resolution, because the viewing distance will be further away than for a normal sized image. For example huge billboards are often printed at 20dpi or less.
100dpi would be sufficient, in fact it's more than enough, you could go even lower to 72dpi
8' x 4' = 96" x 48" @ 100dpi = 9600px x 4800px
The image quality will be perfect if you need to print it smaller. The dpi/ppi is irrelevant.
BTW the resolution of a digital image is measured in pixels, not ppi or dpi. You really need to read this before you go any further: The Myth of DPI http://www.rideau-info.com/photos/mythdp… - understanding what DPI really is, and why it's not the resolution of an image
And if you are getting the image printed digitally then you don't need to give the printer a CMYK image. That's only required for print processes that use separations, such as lithographic printing. Why RGB beats CMYK for digital printing: http://www.printing.org/page/2779 - if the hand outs are being printed using lithographic printing, you should be converting the RGB image only after you created the design - CMYK conversion should be the last thing you do to the image.
And just one more thing, I promise this is the last. If you are adding text to the image, don't do it in Photoshop - Place the photographic (raster) images into Illustrator or InDesign, and set the text there. So that when it gets printed it will be pin sharp - because the text will be vectors, and not pixels.